The present invention relates generally to a method and apparatus for stripping holes of foreign matter to clear the holes, and more particularly to a method and apparatus for stripping holes in a metal substrate having a coating or other foreign matter applied thereto.
One such metal substrate is turbine components. Because turbine components are subjected during normal use to extremely high temperatures as well as significant temperature variations, such components become somewhat distorted, twisted or leans during use, and the normal protective coatings applied to such components become worn. As a result, these components are often repaired by a process that includes first removing the old worn coatings and applying new coatings to the surfaces of the repaired component.
There are known applications in which a metal substrate, such as a turbine component, is formed with a plurality of holes or openings passing through a surface of the component, and these openings must sometimes be cleared or stripped of foreign matter that is lodged in the holes. This removal process is substantially more difficult in situations where the metal substrate also has a coating over an exposed surface of the substrate, and the holes are covered in whole or in part by the coating.
By way of example, it is well known that in gas turbine components holes are formed in the component to extend through an exterior wall of the turbine component to permit a cooling gas to be discharged through the holes from an interior cavity in the component that is in fluid communication with the hole and thereby form a thermal barrier film that protects the turbine component from the high temperature and temperature variations to which the turbine component is exposed during use.
As used herein, the term “turbine components” is intended to include individual turbine blades, blades mounted on a disk, blades that are machined integral to a rotor, air foils, vanes or buckets, shrouds, seals or duct segments, liners and transition pieces.
In most turbine components, one or more layers of coatings are applied to the surfaces of the component during the aforesaid repair process to replace the original coatings after they are removed during the repair process and to protect the underlying base metal from heat, abrasion, and other elements encountered by the component in its normal usage. For example, turbine blades may be formed of a base metal, a bond coating that is often diffusion bonded metallurgically to the base metal, and an outer thermal barrier coating (TBC) that is applied to the bond coat. After a period of usage of the turbine component, these coatings become worn and the component requires must be repaired as discussed above, and this repair process includes removal of the old worn coatings and applying new coatings applied over the surface of the component. It will apparent, however, that if these coatings are applied over the surface of a turbine component that is formed with the holes as described above, the coating or coatings either partially fill or completely fill the holes which will significantly obstruct the flow of cooling gas through the holes and thereby render the holes less effective or useless for their intended purpose unless the holes are stripped or cleared of most if not all of the coating material within the holes.
One known method of dealing with this problem in turbine components is to fill the holes with a plastic like material (e.g. Plastisol) before the coating is applied, and then cure this material using ultra-violet rays. In some cases, enough plastic like material is present so that it extends slightly above the exterior surface of the component so as to cause a bump or protrusion which can be used to locate holes for manual stripping. If the step of adding plastic material results in excess material being left on the exterior surface of the turbine component, which is often the case, such excess must be manually removed. After the holes have been plugged in this manner, a bond coating is applied over the plugged holes and the exterior surface of the component, and then the plastic material in the holes is removed using conventional equipment, and a small hand tool, such as a Dremel device, is manually used to open the holes by clearing any coating material that is lodged in the holes. Next a TBC is applied over the bond coating and, again, the holes are manually cleared using a tool such as a Dremel device. When it is recognized that many turbine components have a very large number of holes that must be manually stripped using this known method, it will be apparent that the process of manually stripping all of such holes is very time consuming and labor intensive. For example, one typical turbine blade will have as many as dozens or hundreds of holes, and a turbine liner may have tens of thousands of holes, whereby it may take an hour or more to manually strip all of these holes in the turbine blade, and hundreds of hours to manually strip the holes in the liner.
Therefore, a need exists for a method and apparatus that is capable of stripping holes from a metal substrate using a machine and a method that avoids the manual, labor intensive process that is presently being used.